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LANGUAGE PROCESSORS
Presented By: Prof. S.J. Soni, SPCE – Visnagar.
Introduction
Language Processing activities arise due to the
differences between the manner in which a software
designer describes the ideas concerning the behavior
of a software and the manner in which these ideas
are implemented in a computer system.
The designer expresses the ideas in terms related to
the application domain of the software. To implement
these ideas, their description has to be interpreted
in terms related to the execution domain.
Semantic Gap
Semantic Gap has many consequences
Large development time
Large development effort
Poor quality of software
Application
Domain
Execution
Domain
Semantic Gap
Specification and Execution Gaps
The software engineering steps aimed at the use of a PL can be grouped into
Specification, design and coding steps
PL implementation steps
Application
Domain
PL
Domain
Specification Gap
Execution
Domain
Execution Gap
Specification and Execution Gaps
Specification Gap
It is the semantic gap between two specifications of the
same task.
Execution Gap
It is the gap between the semantics of programs (that
perform the same task) written in different
programming languages.
Language Processors
“A language processor is a software which bridges a
specification or execution gap”.
The program form input to a language processor as
the source program and to its output as the target
program.
The languages in which these programs are written
are called source language and target language,
respectively.
Types of Language Processors
A language translator bridges an execution gap to the machine language (or assembly language) of a computer system. E.g. Assembler, Compiler.
A detranslator bridges the same execution gap as the language translator, but in the reverse direction.
A preprocessor is a language processor which bridges an execution gap but is not a language translator.
A language migrator bridges the specification gap between two PLs.
Language Processors - Examples
C++
preprocessorC++ Program C Program
Errors
C++
translatorC++ Program Machine Language
Program
Errors
Interpreters
An interpreter is a language processor which bridges an
execution gap without generating a machine language program.
An interpreter is a language translator according to classification.
Application
Domain
PL
Domain
Execution
Domain
Interpreter Domain
Language Processing Activities
Program Generation Activities
Program Execution Activities
Program Generation
Program GeneratorProgram
Specification
Program in
target PL
Errors
Application
Domain
Target PL
Domain
Execution
Domain
Program
Generator
Domain
Specification Gap
Program Execution
Two popular models for program execution are
translation and interpretation.
Program translation
TranslatorSource
Program
Target
Program
Errors
m/c
language
program
Data
A program must be translated before it can be executed.
The translated program may be saved in a file. The saved program may be
executed repeatedly.
A program must be retranslated following modifications.
Program Execution
Program interpretation
PC
Errors
Interpreter
Source
Program
+
Data
Memory
PC
CPU
Machine
Language
Program
+
Data
Memory
Interpretation Program execution
Fundamentals of Language Processing
Language Processing = Analysis of SP + Synthesis of TP
Collection of LP components engaged in analysis a source
program as the analysis phase and components engaged in
synthesizing a target program constitute the synthesis phase.
Analysis Phase
The specification consists of three components:
Lexical rules which govern the formation of valid lexical units in the source language.
Syntax rules which govern the formation of valid statements in the source language.
Semantic rules which associate meaning with valid statements of the language.
Consider the following example:
percent_profit = (profit * 100) / cost_price;
Lexical units identifies =, * and / operators, 100 as constant, and the remaining strings
as identifiers.
Syntax analysis identifies the statement as an assignment statement with percent_profit
as the left hand side and (profit * 100) / cost_price as the expression on the right
hand side.
Semantic analysis determines the meaning of the statement to be the assignment of
profit X 100 / cost_price to percent_profit.
Synthesis Phase
The synthesis phase is concerned with the construction of target language statements which have the same meaning as a source statement.
It performs two main activities:
Creation of data structures in the target program (memory allocation)
Generation of target code (code generation)
Example
MOVER AREG, PROFIT
MULT AREG, 100
DIV AREG, COST_PRICE
MOVEM AREG, PERCENT_PROFIT
…
PERCENT_PROFIT DW 1
PROFIT DW 1
COST_PRICE DW 1
Phases and Passes of LP
Analysis of source statements can not be immediately followed
by synthesis of equivalent target statements due to following
reasons:
Forward References
Issues concerning memory requirements and organization of a LP
Analysis
PhaseSource
Program
Target
Program
Errors
Synthesis
Phase
Errors
Language Processor
Lexical Analysis (Scanning)
It identifies the lexical units in a source statements. It then classifies the units into different lexical classes, e.g. id‟s, constants, reserved id‟s, etc. and enters them into different tables.
It builds a descriptor, called token, for each lexical unit. A token contains two fields – class code and number in class.
class code identifies the class to which a lexical unit belongs. number in class is the entry number of the lexical unit in the relevant table.
We depict a token as Code # no, e.g. Id # 10
Lexical Analysis (Scanning) - Example
i : integer;
a, b : real;
a := b + i;
Symbol Type Length Address
1 i int
2 a real
3 b real
4 i * real
5 temp real
Note that int i first needed to be converted into real, that is why 4th entry is
added into the table.
Addition of entry 3 and 4, gives entry 5 (temp), which is value b + (i *).
The statement a := b+i; is represented as the string of tokens
Id#2 Op#5 Id#3 Op#3 Id#1 Op#10
Syntax Analysis (Parsing)
It processes the string of tokens built by lexical analysis todetermine the statement class, e.g. assignment statement, ifstatement etc.
It then builds an IC which represents the structure of astatement. The IC is passed to semantic analysis todetermine the meaning of the statement.
real
a b
:=
a +
b ia, b : real
a := b + i
Semantic Analysis
It identifies the sequence of actions necessary to implement the meaning of a source statement.
It determines the meaning of a sub tree in the IC, it adds information to a table or adds an action to the sequence of actions. The analysis ends when the tree has been completely processed.
:=
a, real +
b, real i, int
:=
a, real +
b, real i*, real
:=
a, real temp, real
Analysis Phase (Front end)
Scanning
Parsing
Semantic
Analysis
Source Program
Tokens
Trees
Lexical
Errors
Syntax
Errors
Semantic
Errors
IC
Symbol Table
Constants Table
Other tables
IR
Synthesis Phase (Back end)
It performs memory allocation and code generation.
Memory Allocation
The memory requirement of an identifier is computed from its type,
length and dimensionality and memory is allocated to it.
The address of the memory area is entered in the symbol table.
Symbol Type Length Address
1 i int 2000
2 a real 2001
3 b Real 2002
Synthesis Phase (Back end)
Code Generation It uses knowledge of the target architecture, viz. knowledge of
instructions and addressing modes in the target computer, to select the
appropriate instructions.
The synthesis phase may decide to hold the values of i* and temp in
machine registers and may generate the assembly code.
a := b + i;
CONV_R AREG, I
ADD_R AREG, B
MOVEM AREG, A
Synthesis Phase (Back end)
Memory
Allocation
Code
Generation
Symbol Table
Constants Table
Other tables
Target
Program
IR
IC
Fundamentals of Language Specification
PL Grammars
The lexical and syntactic features of a programming
language are specified by its grammar.
A language L can be considered to be a collection of
valid sentences.
Each sentence can be looked upon as a sequence of
words, and each word as a sequence of letters or
graphic symbols acceptable in L.
A language specified in this manner is known as a
formal language.
Alphabet
The alphabet of L, denoted by the Greek symbol ∑
is the collection of symbols in its character set.
We use lower case letters a, b, c, etc. to denote
symbols in ∑
A symbol in the alphabet is known as a terminal
symbol (T) of L.
The alphabet can be represented using mathematical
notation of a set, e.g.
∑ = { a, b, …, z, 0, 1, …, 9 }
where {, “,”, } are called meta symbols.
String
A string is a finite sequence of symbols.
We represent strings by Greek symbols α, β, γ, etc.
Thus α= axy is a string over ∑
The length of a string is the number of symbols in it.
Absence of any symbol is also a string, null string ε.
Example
α= ab, β=axy
αβ = α.β = abaxy [concatenation]
Nonterminal symbols
A Nonterminal symbol (NT) is the name of a syntax
category of a language, e.g. noun, verb, etc.
An NT is written as a single capital letter, or as a
name enclosed between <…>, e.g. A or <Noun>.
It is a set of symbols not in ∑ that represents
intermediate states in the generation process.
Productions
A production, also called a rewriting rule, is a rule
of the grammar.
It has the form
A nonterminal symbol ::= String of Ts and NTs
L.H.S. R.H.S.
e.g. <article> ::= a | an | the
<Noun> ::= boy | apple
<Noun Phrase> ::= <article> <Noun>
Derivation, Reduction and Parse Trees
A grammar G is used for two purposes, to generate
valid strings of LG and to „recognize‟ valid strings of
LG.
The derivation operation helps to generate valid
strings while the reduction operation helps to
recognize valid strings.
A parse tree is used to depict the syntactic structure
of a valid string as it emerges during a sequence of
derivations or reductions.
Derivation
Let production P1 of grammar G be of the form
P1 : A::= αand let β be a string such that β = γAθ, then replacement of A by α in string β constitutes a derivation according to production P1.
Example
<Sentence> ::= <Noun Phrase><Verb Phrase>
<Noun Phrase> ::= <Article> <Noun>
<Verb Phrase> ::= <Verb><Noun Phrase>
<Article> ::= a | an | the
<Noun> ::= boy | apple
<Verb> ::= ate
Derivation
The following strings are sentential forms of LG.
<Noun Phrase> <Verb Phrase>
the boy <Verb Phrase>
<Noun Phrase> ate <Noun Phrase>
the boy ate <Noun Phrase>
the boy ate an apple
sentential
forms
sentence
Reduction
Let production P1 of grammar G be of the form
P1 : A::= α
and let σ be a string such that σ = γAθ, then replacement of α by A in string σ constitutes a reduction according to production P1.
Step String
0 the boy ate an apple
1 <Article> boy ate an apple
2 <Article> <Noun> ate an apple
3 <Article> <Noun> <Verb> an apple
4 <Article> <Noun> <Verb> <Article> apple
5 <Article> <Noun> <Verb> <Article> <Noun>
6 <Noun Phrase> <Verb> <Article> <Noun>
7 <Noun Phrase> <Verb> <Noun Phrase>
8 <Noun Phrase> <Verb Phrase>
9 <Sentence>
Parse Trees
A sequence of derivations or reductions reveals the syntactic structure of a string with respect to G, in the form of a parse tree.
<Sentence> 9
<Noun Phrase> 6 <Verb Phrase> 8
<Article> 1 <Noun> 2
<Article> 4 <Noun> 5
<Verb> 3 <Noun Phrase> 7
the boy ate an apple
Classification of Grammars
Type-0 grammar (Phrase Structure Grammar)
α ::= β, where both can be strings of Ts and NTs.
But it is not relevant to specification of Prog. Lang.
Type-1 grammar (Context Sensitive Grammar)
α A β ::= α π β,
But it is not relevant to specification of Prog. Lang.
Type-2 grammar (Context Free Grammar)
A ::= π, which can be applied independent of its context.
CFGs are ideally suited for PL specifications.
Type-3 grammar (Linear or Regular Grammar)
A ::= t B | t OR A ::= B t | t
Nesting of constructs or matching of parentheses cannot be specified using such productions.
Ambiguity in Grammatic Specification
It implies the possibility of different interpretation of a source string.
Existence of ambiguity at the level of the syntactic structure of a string would mean that more than one parse tree can be built for the string. So string can have more than one meaning associated with it.
Ambiguous Grammar
E id | E + E | E * E
Id a | b | c
Assume source
string is a + b * c
a + b * c a + b * c
a *
b c
+ c
a b
+ *
Eliminating Ambiguity – An Example
Unambiguous Grammar
E E + T | T
T T * F | F
F F ^ P | P
P id
id a | b | c
a + b * c
id + id * id
P + P * P
F + P * P
T + F * F
E + T * T
E * T (?? Ambiguous)
a + b * c
id + id * id
P + P * P
F + F * P
T + T * F
E + T
E (Unambiguous)
E
E
T
F
P
id
T
F
P
id
T
F
P
id+ *
E
T
F
P
id
T
F
P
id
T
F
P
id+ *
E
GTU Examples
List out the unambiguous production rules (grammar)
for arithmetic expression containing +, –, *, / and ^
(exponent).
E E + T | E – T | T
T T * F | T / F | F
F F ^ P | P
P (E) | <id>
Derive string <id> – <id> * <id> ^ <id> + <id>
E E + T
E – T + T
T – T + T
F – T + T
P – T + T
<id> – T + T
<id> – T * F + T
<id> – F * F + T
<id> – P * F + T
<id> – <id> * F + T
<id> – <id> * F ^ P + T
<id> – <id> * P ^ P + T
<id> – <id> * <id> ^ P + T
<id> – <id> * <id> ^ <id> + T
<id> – <id> * <id> ^ <id> + F
<id> – <id> * <id> ^ <id> + P
<id> – <id> * <id> ^ <id> + <id>
E
E T
E T F
T FT
F FF P
P PP
P
id id id id id– * ^ +
*
^
–
+Parse Tree
Abstract Syntax Tree
+
id–
*id
id ^
id id
Another Example
Consider the following grammar:
S a S b S | b S a S | ε
Derive the string abab. Draw corresponding parse
tree. Are these rules ambiguous ? Justify.
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